426 Prof. C. Claus on the Morphology and 



origin. As the Dendrocoele Turbellaria of fresh and salt 

 water exchanged a free existence for a parasitic one and 

 adapted bodily form and structure to the new conditions of 

 life — losing the outer covering of cilia (with the exception of 

 the vestige still remaining during larval existence), while 

 they acquired suckers and organs of adhesion of various kinds 

 — they became Trematodes, which, in connexion with the 

 easier and more favourable nourishment in the body of a 

 host, acquired the power of producing a far more numerous 

 progeny. 



The closer representation of these processes becomes more 

 complicated and difficult owing to the fact that in the case of 

 so large a number of Trematodes, and practically universally 

 among the Distomas, with which we are especially concerned, 

 we have two different hosts between which the life-history of 

 the species is distributed. The one functions to a certain 

 extent as intermediate host, and brings the intruding parasite 

 only up to a certain stage of development ; it conceals in its 

 body the larval form, though even at this stage it may be 

 capable of reproduction. The second host receives the para- 

 site, which has reached it either actively or passively, and 

 brings it to full development and sexual maturity ; it 

 harbours the sexual form. Now are the intermediate hosts 

 — and this is a question which has already been sagaciously 

 propounded by Rud. Leuckart* — tl merely later intruders into 

 the life-history of the Helminthes," or are they " the original 

 genuine hosts, which primitively brought their intestinal 

 worms to sexual maturity, but were subsequently degraded 

 to the position of intermediate hosts, owing to the fact that 

 the life-history of their parasites, through further development 

 and differentiation, has been spread over a larger number of 

 stages ? " The first case would, to make use of E. Hgeckel's 

 noteworthy expression, represent a ccenogenetic, the latter a 

 ])alingenetic condition. In the former redia? and sporocysts 

 would be later developed forms {i. e. than the sexual stages), 

 secondarily and coenogenetically modified through adaptation ; 

 in the latter, on the other hand, they would represent earlier- 

 existing, philetically older, and once sexual forms. Now 

 when Kudolph Leuckart very emphatically selects the second 

 alternative, pointing by way of justification to the fact that 

 at the present time nearly all Entozoa live during the sexual 

 stage in the bodies of Vertebrates, which are undoubtedly of 

 later origin, it seems to me that he has not hit the right nail 

 on the head. Apart from the fact that fish and other aquatic 



* Rud. Leuckart, ' Die Parasiteu des Menscben,' part 2, Bd. i. p. 148. 



